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91.
Flight from the Tyranny of Tin: The Quest for Practical Radical Sources Free from Metal Encumbrances
Paul A. Baguley John C. Walton 《Angewandte Chemie (International ed. in English)》1998,37(22):3072-3082
An urgent search is being made for generic reagents that promote free radical synthetic transformations as means of banishing the toxic threat of organotin hydrides. Although second-generation tin reagents are beguiling, organosilanes and a range of thiocarbonyl compounds are more intrinsically benign. Metal-free radical chain sequences based around cyclohexadiene derivatives are being developed (see reaction scheme), and tetrathiafulvalenes mimic metals and allow a crossover from homolytic to ionic chemistry. Z=alkene. 相似文献
92.
Emmanuel Lacte Bndicte Delouvri Louis Fensterbank Max Malacria 《Angewandte Chemie (International ed. in English)》1998,37(15):2116-2118
Alkylidene-substituted cyclopentane derivatives are formed in high enantiomeric purity by the reaction shown below. A highly diastereoselective radical cyclization is followed by elimination of a β-sulfinyl radical. Interestingly, the addition of the Lewis acid methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) totally reverses the stereochemical outcome of the reaction. E=CO2Me. 相似文献
93.
Ikuo Nakanishi Shinobu Itoh Tomoyoshi Suenobu Shunichi Fukuzumi 《Angewandte Chemie (International ed. in English)》1998,37(7):992-994
The redox behavior of “active aldehydes” 1 − derived from 3-benzylthiazolium salts and simple aldehydes in the presence of a base has been examined with low-temperature cyclic voltammetry and EPR spectroscopy. The highly negative oxidation potentials of 1 − and the spin distribution of the intermediate radicals 1 . indicate that the active aldehyde can act as an efficient electron mediator in thiamin-dependent enzymatic redox systems. 相似文献
94.
Shigekazu Ito Manabu Kikuchi Masaaki Yoshifuji 《Journal of organometallic chemistry》2007,692(13):2761-2767
A kinetically stabilized phosphaalkyne bearing a bulky Mes∗ (2,4,6-tri-t-butylphenyl) group is useful compound to prepare an enormous number of highly stable 1,3-diphosphacyclobutane-2,4-diyls through reactions with a lithium reagent and an electrophile. By utilizing this synthetic protocol, we prepared several non-symmetrical 1,3-diphosphacyclobutane-2,4-diyls in which the substituents on the phosphorus are different. Furthermore, we succeeded in preparation and characterization of novel air-tolerant symmetrical 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls bearing the identical alkyl substituents on the phosphorus atoms. Structures and properties of the 1,3-diphosphacyclobutane-2,4-diyls indicate characters as singlet ground-state carbon centered biradicals. In addition to those biradicals, we succeeded in preparation and isolation of a novel P-heterocyclic air-stable neutral radical as well as a P-heterocyclic cation radical. 相似文献
95.
ArnaudPierre Schaffner Kandhasamy Sarkunam Philippe Renaud 《Helvetica chimica acta》2006,89(10):2450-2461
A tandem radical process involving conjugate addition to an activated alkene followed by allylation is reported. B‐Alkylcatecholboranes, easily available via hydroboration of the corresponding alkenes, were used to generate the initial radicals. These radicals add efficiently to electrophilic alkenes such as phenyl vinyl sulfone, N‐phenylmaleimide, and dialkyl fumarate. In the last step of this one‐pot process, the radical adducts react with the allylic sulfones. The whole process can be considered as a unique and selective coupling of three different alkenes. 相似文献
96.
Plácido García Maximiliano Burgos Paci Thorsten Berends 《Journal of fluorine chemistry》2005,126(6):984-990
The synthesis of the new compound, bis(trifluoromethyl)dicarbonate, CF3OC(O)OC(O)OCF3, is carried out by reduction of bis(trifluoromethyl)trioxidicarbonate with excess of CO at 0 °C. The product is characterized by IR, Raman, 13C and 19F NMR spectroscopy and its properties are compared with those of the other members of the series CF3OC(O)OxC(O)OCF3, x = 0-3. Single crystals are grown at −25 °C and the X-ray diffraction analysis shows the packing of syn-syn rotamers exhibiting C2 symmetry. DFT calculations predict this rotamer as the most stable one and also structural and vibrational data are predicted reasonably well. 相似文献
97.
Eric Léonel Michael Lejaye Sylvain Oudeyer Jean Paul Paugam Jean-Yves Nédélec 《Tetrahedron letters》2004,45(12):2635-2638
The activation of CBr4 and CCl4 by a bimetallic iron/copper couple in acetonitrile is a new, inexpensive, nontoxic and efficient procedure for gem-dibromo- and gem-dichloromethylenation of nucleophilic alkenes. This new route to gem-dihalocyclopropanes involves dihalocarbene species. 相似文献
98.
The 4‐exo cyclizations of two types of carbamoyl radicals onto O‐alkyloxime acceptor groups were studied as potential routes to 3‐amino‐substituted azetidinones and hence to penicillins. A general synthetic route to ‘benzaldehyde oxime oxalate amides’ (= 2‐[(benzylideneamino)oxy]‐2‐oxoacetamides; see, e.g., 10c ) of 2‐{[(benzyloxy)imino]methyl}‐substituted thiazolidine‐4‐carboxylic acid methyl esters 9 was developed (Scheme 3). It was shown by EPR spectroscopy that these compounds underwent sensitized photodissociation to the corresponding carbamoyl radicals but that these did not ring close. An analogous open‐chain precursor, benzaldehyde O‐(benzylaminoacetaldehyde‐O‐benzyloxalyl)oxime, 15 , lacking the 5‐membered thiazolidine ring, was shown by EPR spectroscopy to release the corresponding carbamoyl radical (Scheme 4). The latter underwent 4‐exo cyclization onto its C?NOBn bond in non‐H‐atom donor solvents. The rate constant for this cyclization was determined by the steady‐state EPR method. Spectroscopic evidence indicated that the reverse ring‐opening process was slower than cyclization. 相似文献
99.
Ewa Kornacka Izabela Legocka Gra?yna Przybytniak Jaros?aw Sad?o 《Polymer Degradation and Stability》2006,91(9):2182-2188
The effect of ionizing radiation on poly(siloxaneurethaneureas), comprising the soft phase of poly(dimethylsiloxane) chains and hard urethane segments, was examined by electron paramagnetic resonance (EPR) spectroscopy and gas chromatography (GC). It was found that radical processes in the investigated polymers were determined predominantly by polysiloxane segments. Dehydrogenation proceeded mainly in the terminal groups of dimethylsiloxane sequences, forming methylene radicals, whereas abstraction of methyl group dominated in inner fragments of the polysiloxane chain. However, upon irradiation no change in glass transition temperature of polysiloxane segments was detected by DSC. 相似文献
100.
Ohashi N Fujitake M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(14):3277-3284
Spectroscopic results on the radicals HCSi, CCO, and FeC obtained by studying in detail energy level structures using 0.8 microm diode laser system are reported. Of these radicals, the CCO radical was investigated mainly using Fabry-Perot type diode lasers with inconvenient mode gaps in the early stage of our near-infrared diode laser spectroscopic study of free radicals, and on the other hand, the FeC and HCSi radicals were studied using an external cavity diode laser. For the FeC radical, which is an interesting radical composed of an iron atom having 3d electrons, information on spin-orbit interaction between the triplet electronic ground state and a low-lying singlet electronic excited state is reported somewhat in detail. For the HCSi and CCO radicals, spectral particularities produced by a Renner-Teller interaction and a spin-orbit interaction are described for their high-resolution spectroscopic interest. 相似文献